Final answer:
You are in motion relative to the outside world but at rest relative to the bus due to the concept of relative motion. This state remains until the bus accelerates or decelerates, during which you would feel the forces acting upon your body.
Step-by-step explanation:
If you are sitting still in your seat on a bus that is traveling 100 km/h on a highway, your body is in motion relative to the outside world but at rest relative to the bus. This is due to the concept of relative motion in physics. When the bus is moving at a constant velocity, there are no forces acting on your body to make you feel like you're moving, as long as the bus moves uniformly without sudden starts, stops, or turns. That is why, inside the bus, you and everything else inside seem to be at rest with respect to each other.
However, during acceleration and deceleration, you feel the changes in motion because your body resists changes in speed or direction, as described by Newton's first law of motion, also known as inertia. This is why standing facing sideways in a bus or train can be easier and safer; your body can more easily readjust its mass distribution laterally to counteract the forward or backward forces when the vehicle accelerates or decelerates.